Microwave attenuator with nondistributed type resistors



y 19,1979 J. L. WORCESTER 3,513,417

MICROWAVE ATTENUATOR WITH NONDISTRIBUTED TYPE RESISTORS Filed Aug. 17. 1955 FIG-1 FIGY. 4

FIG 3 24o 240 I50 I50 L 0 t M '6 4 i INVENTOR.

' BYJOHN L. WORCESTER F |0 5 J44 win-5 ATTORNEYS United States Patent 3,513,417 MICROWAVE ATTENUATOR WITH NON- DISTRIBUTED TYPE RESISTORS John L. Worcester, Walnut Creek, Califi, assignor to E-H Research Laboratories, Inc., Oakland, Calif., a corporation of California Filed Aug. 17, 1966, Ser. No. 573,007 Int. Cl. H01p 1/22 US. Cl. 333-81 1 Claim ABSTRACT OF THE DISCLOSURE The present invention is directed to a microwave attenuator, and more specifically to an attenuator which utilizes substantially nondi'stributed type resistors for the attenuating elements.

Generally, when the wavelength of the operating frequency of a circuit approaches the same order of magnitude as the dimension of a passive element, such as a resistor, the element begins to exhibit a distributed characteristic in which inductive and capactive effects are also present. Thus in the microwave field where the wavelength, of course, is very short, elaborate and expensive circuit elements have heretofore been necessary to minimize or control these high frequency side effects. Such methods include the use of waveguides, elements in which the physical configuration is carefully designed, and from a circuit standpoint the careful matching of circuit elements to their associated portions of the circuit.

It is a general object of the present invention to provide an im roved microwave attenuator which eliminates many of the above named disadvantages.

It is another object of the invention to provide a microwave attenuator in which low cost carbon resistors are used for the attenuators elements.

It is still another object of the invention to provide a microwave attenuator in which the problems of matching are minimized.

It is another object of the invention to provide a microwave attenuator which is simple and inexpensive in construction.

In accordance with the above objects, there is provided a microwave attenuator having a predetermined maximum rated power dissipation for attenuating microwave signal energy from an energy source of a predetermined impedance. The attenuator includes a plurality of attenuating network-s, each including series and parallel connected substantially non-distributed type resistors. The networks are series connected in a predetermined sequence, the first network in the sequence having a predetermined power handling capability. Such first network also has nondistributed type resistors which are spaced "ice from a ground plane with a spacing such that the network has a characteristic impedance substantially equal to the predetermined source impedance to which it is connected. Subsequent networks in the sequence may have a characteristic impedance different than the source impedance, and a power dissipation capability less than that of the first network.

Additional objects of the invention will appear from the following description in which the preferred embodiment of the invention has been set forth in detail in conjunction with the accompanying drawing.

Referring to the drawing:

FIG. 1 is a schematic diagram showing the several attenuation networks embodying the invention along with the means by which they are inter-connected;

FIG. 2 is a schematic circuit of a first attenuating network;

FIG. 3 is a schematic circuit of another attenuating network;

FIG. 4 is a schematic circuit of yet another attenuating network; and

FIG. 5 is a perspective view showing how typical resistive elements are located with respect to a ground plane.

The over-all microwave attenuator of the present invention is schematically illustrated in FIGURE 1 where the individual attenuation networks R through R are illustrated as being mounted on a stator 10 and interconnected by a specially constructed rotor 11. The rotor includes U-shaped inter-connector leads and terminals 12 which serve to selectively interconnect, in a series sequence, selected attenuation networks R through R The rotor in the position as shown is series connecting attenuator networks R and R the input lead being connected to the stator 10, and the output to the rotor 11. The rotor itself is detented to move between the attenuation positions indicated by the attenuation ratios, also shown on the drawing. For example, where the input is directly connected to the output, the attenuation ratio is 1/ 1. The fol lowing table illustrates the different amplitude attenuation ratios and the series connected networks which accomplish the attenuations:

Networks in series: Attenuation ratio R t 2/1 R1+R2 5/1 R1+R2+R3 10/1 R1+R2+R3+R4 20/1 R1+R2+R3+R4+R5 50/1 R1+R2+R3+R4+R5+R6 /1 The above attenuation ratios are, of course, either voltage or current, and the power reduction is the square of the ratio. For example, in the case of the attenuation introduced by R the power dissipation is actually threequarters of the total power dissipation of the over-all attenuator, and thus if the maximum rated power dissipation of the microwave attenuator is 50 watts, the R network would require an attenuation power capacitor of approximately 37.5 watts. Similarly, R and R combined have a net attenution of 5/1 in voltage and hence dissipate of the input power. R by itself must be capable of including the difference, or i of the input power. For 50 watts input R must be capable of handling about 10.5 watts.

From the above, it is apparent that if the first network in a series sequence of attenuating networks has a substantial power dissipation capability, the demands on the subsequent networks are greatly reduced.

In accordance with the invention, networks R 'R are composed exclusively of non-distributed type resistors; in other words, the resistors are ordinary low cost carbon type resistors normally used in low frequency applications. The different networks are illustrated in detail in FIGS. 2, 3 and 4, the specific values of resistance being indicated in ohms for each resistor where the specific application is for an attenuator with a 50 ohm impedance. FIG. 2 shows network R FIG. 3 network R and FIG. -4 networks R R R and R Since network R must dissipate over three-quarters of the total rated power dissipation of the microwave attenuator, each individual resistor is rated at 2 watts. Similarly, the R network shown in FIG. 3 has a 2 watt per resistor capacity. The remaining attenuator sections as illustrated by the typical network of FIG. 4 have correspondingly lower power ratings, for example 2 watts per resistor for R 1 watt for R /2 watt for R and R Although the carbon resistors which constitute the different attenuation networks have been characterized as being of a non-distributed type, this applies only to their physical configuration; in other words, special care must still be taken to account for the effects of distributed inductance and capacitance. At the higher microwave frequencies, these effects are present and the individual resistors have an equivalent circuit which includes an inductor in series and a capacitor in parallel with the resistors. Normally, if these effects are not compensated for, the attenuator network Will not have a characteristic impedance of the proper value so as to match it to its source. In addition to the distributed effects of the individual resistors, when the resistor is put into a circuit, the ground plane must be considered since there will be a capacity between each resistor and ground. Moreover, where several low power resistors are used in a network to yield an over-all relatively high power dissipation capability, the entire resistive network tends to have a continuous distribution, and must be treated as a transmission line. Thus, to achieve a proper characteristic impedance, the theoretical transmission line requirements must also be fulfilled. For example, the characteristic impedance of a lossy transmission line is given by:

Ml +j where R, G, L and C are respectively the series resistance, shunt conductance, series inductance, and shunt capacity per unit length of the transmission line. In order to construct an attenuator where the characteristic impedance, Z,,, is independent of frequency, it is well known that the following equality must be maintained:

In general, the following considerations must be taken into account when physically arranging non-distributed type resistors with reference to a ground plane to provide for a large power dissipation, yet maintaining a frequency independent characteristic impedance:

(1) To reduce the equivalent series inductance of each resistor, a parallel circuit arrangement may be used. This, however, causes an attendant increase in the end to end or shunt capacity of the resistor. However, capacity is a secondary effect since the resistive elements which are connected in parallel, such as the 10 ohm, ohm, 24 ohm and 68 ohm resistors (FIGS. 2 and 3) are of a relatively low magnitude so that the resistance acts as an effective shunt to this end to end capacity.

(2) In order to decrease end to end capacity, where it is critical, (e.g., where resistance is relatively high) resistors may be arranged in series. This will increase series inductance which, however, is less crucial because of the 4 already relatively high value of the resistance. In FIG. 2, network R has several relatively high value resistors in series; for example the 270, 240 ohm resistors.

(3) Both series inductance and shunt capacity to ground is affected by the positioning of the resistors wlth respect to the ground plane; a resistor close to the ground plane has its equivalent series inductance reduced and shunt capacity to ground increased. Thus by proper spacing the requirements of Equation 2 can be fulfilled.

FIG. 5 illustrates the placement of resistors such as the 10 ohm, 15 ohm, and 24 ohm resistors in FIG. 2, and the 62, 68 ohm resistors in FIG. 3 with respect to a ground plane 16 and to each other. Because of the relatively low resistance values of these resistors, inductance is a significant factor and therefore locating them properly with respect to a ground plane decreases this series inductance. FIG. 5 represents ideal optimum spacing of these low value parallel connected resistors of FIG. 2 or FIG. 3 from ground plane 16. In the specific embodiment of the invention, it has 'been found that a /8 of an inch spacing for each resistor from the ground plane and a A: of an inch spacing, between resistors provides an optimum characteristic impedance.

On the other hand, the resistors of FIGS. 2 and 3 which are indicated as going to ground, must be located as far as possible from the ground plane since their relatively high resistance can be shunted to ground relatively easily.

The actual spacing of the resistors, both in the series and shunt arms will depend upon the number of resistors in the arm, which is determined by the net resistance of the arm, which is related to the power dissipation requirements.

Thus, by proper spacing from a ground plane, the networks R and R have a characteristic impedance substantially equal to the predetermined source impedance to which they are to be connected. However, it is not necessary that the remaining networks R R shown in FIG. 4, have a characteristic impedance equal to the source impedance. While variations of characteristic impedance Z with frequency do affect transmission properties of a line, small mismatches are most noticeable because of the reflections they introduce. This results in a large VSWR, (Voltage standing-wave ratio) which can be a series problem in many microwave applications. Because R is buffered by R and R by R and R etc., subsequent attenuation elements need not be matched as carefully as the preceding element, at least as far as the VSWR is concerned. For example, any voltage reflection off of R because of a mismatch is effectively attenuated by a factor of 4 by R for the attenuation ratios designated in FIG. 1.

The microwave attenuator of the present invention has been constructed and performs satisfactorily with input pulses having a rise time of 1x l 0 seconds; such a rise time implies a frequency response of from DC to approximately 300 megacycles per second.

Thus, in summary, the present invention provides a microwave attenuator where low cost carbon resistors may be used, herein termed substantially non-distributed type resistors while still minimizing reflections between the attenuator and the energy source with which it is associated. This is accomplished by providing a series connected sequence of attenuator networks, the first network having a relatively substantial power dissipation capabi1- ity with respect to the remaining networks, and having a characteristic impedance which is primarily controlled by means of proper spacing of resistors from the ground plane, and secondarily by use of proper connections and resistive values.

I claim:

1. In a microwave attenuator having a predetermined maximum rated power dissipation for attenuating microwave signal energy from an energy source of a predetermined impedance, a plurality of attenuating networks each including series and parallel connected substantially nondistributed type resistors, rotary switch means for series connecting said networks in a predetermined sequence, said first network in such sequence having a predetermined power dissipating capability greater than one half of said maximum rated power dissipation and having nondistributed type resistors spaced from a ground plane with a spacing such that said network has a characteristic impedance substantially equal to said predetermined source impedance, all subsequent networks in said sequence after the second network having a characteristic impedance dilferent from said source impedance and a power dissipation capability substantially less than said first network.

References Cited UNITED STATES PATENTS 2,247,554 7/1941 Garity et a1. 333-81 2,286,029 6/ 1942 Van Beuren 33381 2,493,653 1/1950 Bowyer-Lowe et a1. 333-81 X 2,760,170 8/1956 May.

2,820,952 1/ 1958 Hancock et a1.

6 3,015,790 1/1962 Eisaman et a1. 323-74 X 3,192,493 6/1965 Bostick.

FOREIGN PATENTS 227,752 4/ 1960 Australia. 628,73 7 9/ 1949 Great Britain. 644,066 10/ 1950 Great Britain.

795,415 5/1958 Great Britain.

OTHER REFERENCES Beatty, Robert W.: Cascade-Connected Attenuators, Journal of Research of the National Bureau of Standards, September 1950, vol. 45, #3, pp. 231235.

Schafer, G. E., and Rumfelt, A. Y.: Mismatch Errors in Cascade-Connected Variable Attenuators, IRE Transactions on Microwave Theory and Techniques, MTT- 6-7, 1958-1959, pp. 447-453.

HERMAN K. SAALBACH, Primary Examiner 20 W. H. PUNTER, Assistant Examiner U.S. Cl. X.R. 323-79 

